1. Field of the Invention
The present invention relates to a capacitance type detector. More specifically, the present invention relates to a capacitance type detector for near contact or contact detection of an object contained in a container, such as a gas or a powder and granular material, using dielectric constant of the object as a capacitance value.
2. Description of the Background Art
Conventionally, a capacitance type detector has been used when a level of an object, for example, is to be detected, as the capacitance type detector is applicable to wide range of objects to be detected including liquid, powder, sediments in a liquid, water and oil and it is easy to install and has no variable unit.
The wide range of applicable objects, however, is realized by changing a structure of an electrode portion of the detector, or by changing components including addition or exchange of components of a circuitry, for example. Further, as the capacitance is measured based on the dielectric constant of a substance, the detector is susceptible to environmental capacitance such as the capacitance of the container. Accordingly, on-site calibration is necessary before detecting operation when the detector is set in a container, which is quite troublesome. Further, detector operation may be less stable when moisture percentage of the object changes because of moisture or when a small amount of a conductive object happens to be adhered on the detector.
In view of the above described problems, Japanese Patent Laying-Open No. 59-54928 describes a capacitance type material level indicator. The level indicator includes a constant oscillation circuit, an amplifier circuit, a resonance circuit having an electrode body of which capacitance changes in accordance with the object of detection, a phase comparing circuit, an automatic calibration circuit having a switch, and an output circuit. An oscillation signal from the constant oscillation circuit is supplied as an excitation signal through the amplifier circuit to the resonance circuit, phase difference between the oscillation signal and the excitation signal of which phase has been changed by the resonance circuit is detected by the phase comparing circuit, and a detection signal corresponding to the phase difference is applied to the automatic calibration circuit. When the switch is pressed, the automatic calibration circuit changes stepwise the operational characteristic of the resonance circuit so that the detection signal matches a reference signal. Further, application to the object and the calibration operation are facilitated, as the operation which takes place is to change the reference signal so that the reference signal matches the detection signal.
In this method, however, calibration is still necessary, though it is simply a pressing of the switch at the site. When an operator fails to calibrate, correct operation is not ensured. When it becomes necessary to change the place of setting, calibration is necessary every time. Further, it is difficult to utilize the resonance state of the resonance circuit, and therefore detection sensitivity may vary.
Japanese Patent Laying-Open No. 5-87612 describes another example, which is a liquid level detector using a PLL circuit. In this example, a PLL circuit is constituted by a phase comparing circuit, a loop filter (integrating circuit) and a voltage controlled oscillation circuit (VCO circuit), in which a pre-oscillation circuit is oscillated at an oscillation frequency in accordance with the object of detection, the oscillation frequency signal is input to the phase comparing circuit of the PLL circuit, and the frequency is compared with the frequency signal of the VCO circuit, so as to detect the object.
In the second example, however, any of the following predetermined conditions must be satisfied.
(1) When there is no object of detection, the oscillation frequency of the pre-oscillation circuit must be out of the lock range of the PLL circuit, and when the object comes to be in contact or in the vicinity, the oscillation frequency of the pre-oscillation circuit must be set within the locked range of the PLL circuit. PA1 (2) When there is no object of detection, the oscillation frequency of the pre-oscillation circuit must be within the locked range of the PLL circuit, and when the object is brought into contact or in the vicinity, the oscillation frequency of the pre-oscillation circuit must be set outside the locked range of the PLL circuit. PA1 (3) When there is no object of detection and when the object is brought into contact or in the vicinity, the oscillation frequency of the pre-oscillation circuit must be set within the locked range of the PLL circuit.
When the range of variation of the frequency of the pre-oscillation circuit and the locked range of the PLL circuit are to be set to satisfy the limited conditions, the site and the object must be taken into consideration, and therefore, setting is very difficult. In other words, applicable range may be limited.